1. Field of the Invention
The present invention relates to an optical element contamination preventing method and an optical element contamination preventing device that prevent optical elements from contamination with a scattered material generated together with extreme ultraviolet light (EUV) in an EUV light source apparatus used as a light source for exposure devices.
2. Description of the Related Art
The transition to microstructures in semiconductor processes has recently been followed by a rapid transition to microstructures in photolithography, and next-generation processes have created a demand for microprocessing at a level from 100 nm to 70 nm and further for microprocessing at a level of 50 nm or less. Accordingly, for example, the development of exposure devices that combine a EUV light source with a wavelength of about 13 nm and a catadioptric system is expected, such exposure devices meeting the requirement for microprocessing at a level of 50 nm or less.
EUV light sources of three types are known: an LPP (laser produced plasma) light source (referred to hereinbelow as an LPP-type EUV light source apparatus) that uses plasma generated by irradiating a target with a laser beam, a DPP (discharge produced plasma) light source that uses plasma generated by an electric discharge, and an SR (synchrotron radiation) light source that uses synchrotron radiation. Among them, an LPP light source is thought to be effective as a light source for EUV lithography that requires a power of several tens of watts or higher because this light source has the following advantages over the other light sources: a very high luminance close to black body radiation can be obtained because the plasma density can be significantly increased; light emission only in the necessary wavelength band can be obtained by selecting a target substance; no structural elements such as electrodes are present around the light source because a point light source having an almost isotropic angular distribution is used; and a very large collection angle of 2πsteradian can be ensured.
The principle of EUV light generation in the LPP system will be explained below. Where a target substance supplied into a vacuum chamber is irradiated with a laser beam, the target substance is excited and converted into plasma. A variety of wavelength components including the EUV light are emitted from the plasma. An EUV collector mirror that selectively reflects the desired wavelength component (for example, a component having a wavelength of 13.5 nm) is disposed within the vacuum chamber, the EUV light is reflected and collected by the EUV collector mirror, and the collected light is outputted to an exposure device. Tin (Sn), lithium (Li), xenon (Xe), and the like can be used as the target substance, but tin (Sn) is preferred among them because it allows a high EUV conversion efficiency to be obtained. A multilayer film (Mo/Si multilayer film) in which molybdenum (Mo) thin films and silicon (Si) thin films are alternately laminated is formed on the reflecting surface of the EUV collector mirror.
In such LPP-type EUV light source apparatus, problems are associated with the effect produced by neutral particles and ions emitted from the plasma and target, in particular, when a solid target is used. Because the EUV collector mirror is disposed close to plasma, neutral particles emitted from the plasma and target adhere to the reflective surface of the EUV collector mirror and decrease the reflectance of the mirror. On the other hand, ions emitted from the plasma erode (in the present application, this process will be referred to as “sputtering”) the multilayer film formed on the reflective surface of the EUV collector mirror. In the description of the present application, the adverse effect produced by such neutral particles and ions on optical elements is called “contamination”. The scattered material from plasma containing the neutral particles or ions and residual fragments of the target substance are called “debris”.
In an EUV collector mirror, a high surface flatness, for example, of about 0.2 nm (rms) is required to maintain a high reflectance, and meeting such a requirement is very expensive. Where the EUV collector mirrors are frequently replaced to resolve this problems, not only the maintenance time extends, but also the operation cost rises. Accordingly, from the standpoint of reducing the operation cost of exposure device and shortening the maintenance time, it is desirable that the service life of EUV collector mirror be extended. The mirror life in an EUV light source apparatus for exposure is defined, for example, as a period in which the reflectance decreases by 10%, and a service life of at least 1 year is required.
As described hereinabove, debris adheres to the surface of the EUV collector mirror and form a metal film. Because the metal film absorbs EUV light, the reflectance of the EUV collector mirror decreases. Assuming that light transmittance of the metal film is about 95%, the reflectance of the EUV collector mirror becomes about 90%. For the service life of EUV collector mirror to be equal to or more than 1 year, the decrease in the reflectance of the EUV collector mirror with respect to the EUV light having a wavelength of 13.5 nm has to be within 10%. Therefore, the allowed values of the adhered quantity (thickness) of the metal film on the reflective surface of the EUV collector mirror are extremely small and constitute about 5 nm for lithium and about 0.75 nm for tin.
Because metal films of such thickness are formed within a comparatively short period, it is important to prevent the adhesion of metal film to the EUV collector mirror. A variety of methods disclosed in the patent documents and the non-patent documents mentioned below have been suggested to prevent the adhesion of metal film.
The patent document 1 (US Patent Application Publication No. 2005/0279946 (Specification, page 1)) discloses a technology for generating a magnetic field or an electric field within a vacuum chamber and guiding the debris. Where the desired magnetic field or electric field is generated within a vacuum chamber, ions that are scattered from plasma toward optical elements are deflected and guided to locations other than the optical elements.
However, the technology described in the patent document 1 is effective only with respect to ions contained in the debris. The debris, however, contains not only ions, but also neutral particles. The neutral particles, which carry no electric charge, are not deflected by the magnetic field or electric field and reach the optical elements.
The patent document 2 (U.S. Pat. No. 6,987,279 (Specification, page 1)) discloses a method by which neutral particles emitted from plasma are ionized by an appropriate means such as ultraviolet radiation and then deflected by the action of a magnetic field. The patent document 3 (Japanese Patent Application Laid-open No. 2006-80255) discloses a method similar to that of the patent document 2 by which neutral particles emitted from plasma are ionized and deflected by the action of a magnetic field. In the patent document 3, electron cyclotron resonance (ECR) is induced by irradiating electrons with microwaves, and neutral particles are ionized by causing the plasma to collide with neutral particles. With the inventions described in the patent document 2 and the patent document 3, it is possible to deflect not only ions emitted from plasma, but also neutral particles.
However, neutral particles with a large diameter are difficult to ionize. Therefore, large neutral particles are not deflected by a magnetic field and reach optical elements.
The non-patent document 1 (F. Bijkerk, E. Louis, M. van der Wiel, G. Turcu, G. Tallents, and D. Batani, “Performance Optimization of a High-Repetition-Rate KrF Laser Plasma X-Ray Source for Microlithography”, J. X-Ray Sci. Technol., 3, 133-135 (1992)) and the non-patent document 2 (G. D. Kubiak, D. A. Tichenor, M. E. Malinowski, R. H. Stulen, S. J. Haney, K. W. Berger, L. A. Brown, J. E. Bjorkholm, R. Freeman, W. M. Mansfield, D. M. Tennant, O. R. Wood II, J. Bokor, T. E. Jewell, D. L. White, D. L. Windt, and W. K. Waskiewics, “Diffraction-limited soft x-ray projection lithography with a laser plasma source”, J. Van. Sci. Technol. B9, 3184-3188 (1991)) disclose a method for supplying a background gas with a predetermined pressure inside a vacuum chamber. Where a He background gas atmosphere with a pressure of about 0.2 Torr is obtained within a vacuum chamber, the kinetic energy of debris with a diameter of 0.3 μm or less, from among the debris scattered from plasma toward optical elements, can be reduced. This phenomenon can be explained as follows. The debris with a small diameter has a small mass and, therefore, a small kinetic energy (½ MV2) and such particles lose their kinetic energy before reaching the optical elements due to collisions with particles of background gas.
However, debris with a diameter of 0.5 μm or more, such as described in the non-patent document 3 (G. D. Kubiak, K. W. Berger, S. J. Haney, P. D. Rockett, and J. A. Hunter, “Laser Plasma Sources for SXPL: Production and Mitigation of Debris” in Soft X-Ray Projection Lithography, A. Hawryluk and R. Stulen, eds., Vol, 18 of OSA Proceedings Series Optical Society of America, Washington, D.C., 1993) and the non-patent document 4 (H. A. Bender, A. M. Eligon, D. O'Connell, and W. T. Silfvast, “Avenger velocity distribution measurements of target debris from a laser-produced plasma”, in Applications of Laser Plasma Radiation, M. C. Richardson, ed., Proc. Photo-Opt. In-strum. 2015, 113-117 (1994)), has a large mass and, therefore, a high kinetic energy. For this reason such debris does not lose their kinetic energy on collisions with background gas particles and, therefore, reaches optical elements.
The patent document 4 (International Patent Application Publication No. 2004/092693 Pamphlet (pages 1 and 11, FIGS. 2A and 2B)) describes a method according to which a debris shield is provided between a plasma generation region and an EUV collector mirror to protect the EUV collector mirror from the scattered debris.
However, with such method, the debris shield is exposed instead of the EUV collector mirror to plasma. As a result, the debris shield is sputtered by high-velocity ions, new debris is generated, and this debris can adhere to the EUV collector mirror. In other words, the debris shield itself becomes a source of debris. Further, frequent cleaning is necessary to remove the debris that has adhered to the debris shield and problems are associated with maintenance.
The non-patent document 5 (Proc. of SPIE, Vol. 5751, p. 248-259) discloses a method by which when a target is from lithium, a mirror is maintained at a high temperature of about 400° C. and the adhesion of debris is prevented by a diffusion effect (evaporation) when the target is from lithium. However, because tin has a large particle diameter and low vapor pressure, tin cannot be caused to diffuse in vacuum.
The debris shield disclosed in the patent document 4 requires frequent maintenance and, therefore, rises the maintenance cost. Further, because the exposure operation has to be stopped each time maintenance is performed, the exposure efficiency is decreased.
The method disclosed in the non-patent document 5 is effective when lithium having a high vapor pressure is used for the target, but is ineffective when the target is from tin having a low vapor pressure.
In general, it can be concluded that the methods disclosed in the patent documents 1-3, and the non-patent documents 1-2 are more effective in preventing the adhesion of debris. Although the drawback of these methods is that debris with a large diameter cannot be prevented from adhering to optical elements, at present the adhesion of such debris has to be tolerated.
The present invention has been created in view of the foregoing and it is an object thereof to prevent the debris emitted together with EUV light from plasma generated by excitation of a target in a chamber by a laser beam from adhering to optical elements provided within the chamber and forming a metal film and to extend the service life of the optical elements.